Questoris versus Mechanicum Knights

What is the difference between Questoris Knights and Mechnicum Knights in the Horus Heresy?

I've been asked this question a small number of times, but thought I would save it up until I reviewed the Knights out of Liber Mechanicum to make a quick summary.

Background.

In terms of the lore, the difference is remarkably straight forward. The Questoris Knights have sworn loyalty to the Imperium of Man - or rather are the more adventure seeking, plunder happy, wanna be Mechanicum escapees made of loyal noble households and able to wage their own wars should they feel the need and are generally loyal to the original ideals of Unity (but not necessarily, because, you know, Heresy). The Mechanicum Knights serve the Mechanicum directly by contrast.  

Unit Rules.

The stat lines of the common knights are identical as you might expect (i.e., the Questoris Knight Magaera and the Mechanicum Knight Magaera are identical). For this reason, I don't think I will be reviewing each unit distinctly! However, there are a few special rules different between the two. 

Mechanicum gets IWND at 6+ and an added Flare Shield. These are not bad upgrades overall, but the Mechanicum variants do cost slightly more than the Questoris versions. 

Faction Rules.

The biggest and most obvious difference comes from the faction rules. The Questoris Knights get everything to do with the Household ranks, but at the cost of having to field 2 Troops for every Lord of War -- which is to say you have to have Armigers. Not ever Moirax. It has to be Armigers. This isn't all bad since Armigers can serve good roles as anti-marine shooters at long range (in a pinch this can also hurt vehicles with lucky rolling), and melta killers at shorter range.